Grain sorghum response to granular formulations of iron sources and hydrophilic polymers

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Abstract
Banded hydrogels containing FeSO4 have been shown to provide available Fe for plants on calcareous, Fe‐deficient soils. Because granular mixtures of gel‐forming polymers and Fe sources would be easier to apply to soil than hydrogels, several such granular products were compared with banded hydrogels in greenhouse pot experiments. Several FeSO4 + gel‐forming polymer (polyacryl‐amide) formulations were granulated in a rotary tableting press and band applied to an Fe‐deficient Epping silt loam soil (Ustic Torriorthent). None of the granular formulations of FeSO4 + polymers banded alone or with other additives (urea, ammominum sulfate, or bentonite clay) was effective in providing available Fe to grain sorghum (Sorghum bicolor L. Moench). However, a granular formulation of FeEDTA + polymer increased sorghum dry matter yields and Fe uptake. Banded hydrated polymer formulations containing either FeEDTA or FeSO4 also were effective in providing available Fe to sorghum. Results of an associated laboratory experiment showed that some gel formation occurred at granule sites of the FeSO4 + polymer formulations within one week after soil application. Apparently, gel formation was inadequate to affect Fe reactions with the soil to form unavailable products. Limited gel formation with granular products may have resulted from insufficient free water in soil, high solute concentrations in the soil solution surrounding the granules, or hard granules. These results suggest that application of granular products of FeSO4 with polymers may not be an effective method of soil application unless other polymers, additives, or ratios of polymers, additives, and FeSO4 can be identified.